Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is a protein kinase A (PKA) activated epithelial anion channel. It's responsible for transportation of salt and fluid in many organs through the apical membrane of epithelial cells. CFTR regulates fluid and electrolyte balance in epithelial tissues, such as in the lungs, sinuses, pancreas, intestine, reproductive system, and sweat glands (Zielenski, J., Genotype and phenotype in cystic fibrosis, Respiration, 2000, 67(2), 117-133).
Over 2000 mutations in the CFTR gene have been identified and the majority are extremely rare (Sosnay, P. R. et al., Defining the disease liability of variants in the cystic fibrosis transmembrane conductance regulator gene, Nat. Genet. 2013, 45(10), 1160-1167). ΔF508 is the most common CFTR mutation worldwide. Up to 91% of patients with CF have the ΔF508 mutation on at least one allele. There are 11 mutations occur at a frequency of >1% globally: G542X, G551D, R117H, N1303K, W1282X, R553X, 621+1G→T, 1717-1G→A, 3849+10kbC→T, 2789+5G→A, and 3120+1G→A. The mutations in the CFTR gene can cause disruptions at various stages of CFTR protein synthesis or in several aspects of CFTR protein function. They can result in less CFTR protein at the cell surface, virtual absence of CFTR protein, or dysfunctional CFTR protein at the cell surface (Zielenski, J., Genotype and phenotype in cystic fibrosis. Respiration, 2000, 67(2), 117-133).
Cystic fibrosis (CF), a systemic, multiorgan disease, is caused by loss of CFTR protein-mediated ion transport. Defective ion transport leads to an imbalance of fluid and electrolytes causing thick, sticky mucus and viscous secretions to accumulate in different organs; the defection interferes with the proper function of organs and causes diseases in lungs (such as cough, viscous sputum, dyspnea, chronic endobronchial infections and inflammation, bronchiectasis, and end-stage lung disease) (Cystic Fibrosis Foundation. CFF Patient Registry, Bethesda, Md., 2014; Knowles, M. R. et al., Mucus clearance as a primary innate defense mechanism for mammalian airways, J. Clin. Invest., 2002, 109, 571-577), pancreas (such as pancreatic insufficiency, nutrient and fat malabsorption, vitamin deficiency, acute/chronic pancreatitis, and CF-related diabetes mellitus) (Bronstein, M. N., Pancreatic insufficiency, growth, and nutrition in infants identified by newborn screening as having cystic fibrosis, J. Pediatr. 1992, 120, 533-540), gastrointestinal system (gastroesophageal reflux disease, distal intestinal obstructive syndrome, biliary duct obstruction, focal biliary cirrhosis, and chronic constipation) (Dray, X. et al., Distal Intestinal Obstruction Syndrome in Adults With Cystic Fibrosis, Clin. Gastroenterol. Hepatol., 2004, 2, 498-503; De Boeck, K., et al., Pancreatitis among patients with cystic fibrosis: correlation with pancreatic status and genotype, Pediatrics, 2005, 115, e463-e469), sinus (nasal congestion loss of smell, sinusitis, chronic infection, and nasal polyps) (Wang, X., et al., Mutation in the Gene Responsible for Cystic Fibrosis and Predisposition to Chronic Rhinosinusitis in the General Population, JAMA., 2000, 284(14), 1814-1819), reproductive system (infertility and congenital bilateral absence of vas deferens) (Cystic Fibrosis Canada, Sexuality, Fertility and Cystic Fibrosis for Adults), and the sweat glands (such as excessive salt loss, dehydration, chronic metabolic alkalosis, heat prostration, and high levels of sweat chloride) (Quinton, P. M., Cystic Fibrosis: Lessons from the Sweat Gland, Physiology, 2007, 22, 212-225). Symptoms of CF manifest throughout life with great variability among patients, though lung disease is the primary cause of mortality (O'Sullivan, B. P. et al., Cystic fibrosis. Lancet. 2009, 373, 1891-1904). As a result, compounds that can modulate the CFTR activities and restore or enhance the function of mutant and wild type CFTR may be used to treat the above diseases.
The effect of modulating CFTR activities on inflammatory or obstructive airways diseases or mucosal hydration may be measured by determining the chloride ions' movement in cell-based assays (Hirsh, A. J. et al., Evaluation of Second Generation Amiloride Analogs as Therapy for Cystic Fibrosis Lung Disease, J. Pharmacol. Exp. Ther., 2004, 311(3), 929-938; Moody, C., et al., Inositol polyphosphate derivative inhibits Na transport and improves fluid dynamics in cystic fibrosis airway epithelia, Am. J. Physiol. Cell Physiol., 2005, 289(3), C512-0520).